I used them for years for (1) close-ups and (2) teles > 135mm. My favorite setup was the slide-focus 400/6.8. On the slow side, but light, fast and exceptionally sharp images.
How do they work...they work very well, but in general more slowly than an SLR, and metering through the lens, although possible, makes the whole process even slower. For close-up work, this isn't a big factor. For tele lenses, it's ok as long as the light's not too tricky. Bear in mind that pointing a reflected light meter downrange and metering, say, 30-45 degrees of the scene will, under some circumstances, produce a reading that's not representative of what your 400mm lens, with its very selective field of view, will be capturing. Having said all that, I used it mostly on an M3 and an M4-P and I got along very well indeed.
Lenses...you could mount special macro lenses, as well as 35mm and 50mm RF lenses - only for closeup work, they wouldn't focus to infinity because of the thickness of the Viso mirror housing.
There was a 65mm f/3.5 that was a great lens and would mount in an accessory focusing mount and would focus to infinity.
There were also accessory focusing mounts for 90mm and 135mm lenses that would allow infinity focus, but the tele system came into its own with dedicated tele lenses of 200, 280, 400 and 560mm (in the more recent versions; there were other focal lengths in earlier production). The 400 and 560 were optical twins of their R equivalents and were (still are) exceptional lenses.
The range of bits and pieces that supported this system is incredible (in number and diversity). However, unless you really want to experience it, or unless you get such a charge out of using these beautifullyl engineered pieces, you're way ahead buying a good SLR and the proper macro lens. (OK, I will stand still now while you all throw things at me.)
